Recently, various studies have enthusiastically been conducted on the practical use of transportation equipment that utilizes hydrogen, natural gas and the like as energy. To facilitate the practical use, it is required at the same time to fulfill infrastructure of a usage environment for storing and transporting such gases at high pressure, so that high-strength materials for use in this usage environment, having tensile strength of more than 800 MPa, have simultaneously been developed as well as planning of applications of these materials.
In such a background, as material for use in the above environment, Patent Documents 1 to 3 suggest, for example, austenitic stainless steel having high content of Mn for the purpose of enhancing solubility of N, and also containing V or a combination of V and Nb so as to utilize solid-solution strengthening of N and precipitation strengthening of Nitride, thereby attaining high strengthening.
In the case of using such high-strength austenitic steel containing a large amount of N as a structure, assembly by welding is necessary, and welded portions are required to have strength equal to that of base metal in the light of a use performance thereof. For this purpose, Patent Documents 3 to 5 suggest welding materials (weld metal) having tensile strength of more than 800 MPa by positively utilizing Al, Ti, and Nb, for example.
Unfortunately, both the welding materials and the weld metal produced by using these welding materials require postweld heat treatment for attaining high strength. Meanwhile, taking account of a real size of a large structure, postweld heat treatment for such long hours greatly limits its feasibility, and may also cause extreme increase in production cost.